The invention relates to a method and apparatus for venting an enclosed housing to maintain a positive known reference pressure in the housing in relation to the variable pressure atmosphere surrounding the housing. In many industrial applications it is often desirable to maintain a positive known reference pressure within an enclosed housing, for example a piece of machinery, in order to exclude the surrounding atmosphere from the machinery. The problem also occurs when delicate instruments are lowered into boreholes that are drilled to produce petroleum deposits. In this case the borehole fluids must be excluded from the housing containing the instruments.
One procedure for excluding the atmosphere from an enclosed space is to fill the enclosed space with a purge fluid whose pressure slightly exceeds the surrounding atmosphere. A simple method for maintaining a positive pressure in an enclosed space is to supply the space with a constant flow of purge fluid and constantly vent the fluid from the enclosed space to maintain the positive known reference pressure. The enclosed space has traditionally been vented using various mechanical-type pressure valves that respond to the pressure of the atmosphere surrounding the enclosed space to control the pressure within the space. While these are satisfactory under some conditions, they are unreliable under varying pressures and temperatures. This is a particular problem in the case of instruments lowered into boreholes where the pressure at the bottom of the borehole may be several hundred pounds in contrast to a surface pressure of substantially zero pounds and the temperature at the bottom of the borehole may be several hundreds degrees F in contrast to a surface temperature of a few tens of degrees F. Mechanical devices have been unreliable under these varying conditions and other means have been used in an attempt to maintain a positive known reference pressure in the instrument housing.
Additional problems also arise as a result of the limited space in tools lowered in the borehole for mounting large mechanical valves. Most mechanical valves are large and contain movable parts that tend to malfunction in a borehole environment. All such valves have inherent hystersis problems.